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Will Wearing Dark Workout Clothes Make Me Too Hot?

I’m about three minutes away from calling the fire department

What a difference a day makes. Just one partially cloudy day prior, you’d finished your usual seven-mile run in record time. Today, you find yourself dragging harder than a farming plow pulled by a jackass, and then you find yourself feeling like a jackass for deciding to go out for your morning run wearing your jet-black Old Navy activewear.

“That must be it!” you insist. “It’s warmer this morning. There isn’t a cloud in the sky. And my black shirt got hot and it tired me out! There’s no way I got so much slower overnight!”

It’s a convenient excuse to cling to, and based on the fraction of knowledge you’re able to recall from your grade-school physics courses, it sounds like it might even be a rare justification for failure that’s rooted in legitimate science.

Fortunately, you won’t need to dust off any of your science books to determine whether or not your embarrassing loss of endurance was owed to a poor selection of training attire. You can simply open your web browser to a search engine and start re-learning thermodynamic laws, and how they conspired with your T-shirt to disrupt your morning cardio session.

Does my choice of clothing really make me warmer?

As you learned (hopefully) in your third grade general science class, white-colored objects reflect 100 percent of the light that shines upon them, because white indicates the absence of all color, so no light is being absorbed by a white surface. Conversely, shirts sporting bright, non-white colors are reflecting a majority of the light that shines upon them back at whomever is viewing them, which is why those colors appear to be bright. 

The darker the clothing appears, the more light it absorbs, and the less light it reflects. Since black indicates the presence of all colors, all of the light shining down upon it is absorbed, thereby causing black objects to absorb the largest amount of light out of all the colors in the spectrum. Because light is energy and it’s forced to assume another form once it’s absorbed, your black shirt transforms 100 percent of the light absorbed by it into heat.

How much hotter do dark colors actually get?

The answer to this question is heavily reliant upon the material being struck by the light. If you’ve ever been unfortunate enough to sit on the hood of a hot vehicle with too much of your skin exposed, you know just how excellent of a thermal conductor metal can be. In one instance, where two Toyota Highlanders sitting side-by-side in the same car lot had their surface temperatures tested, and the only difference between them was that one was coal black and the other was pitch white, the surface of the black vehicle was 46 degrees hotter (159 degrees) than the surface of the white vehicle (113 degrees).

When a similar experiment was conducted involving T-shirts that were left exposed to the sun during a 105-degree summer day, the white T-shirt registered a temperature of 107 degrees while the black T-shirt was significantly warmer at 130 degrees. Yes, the white shirt is two degrees warmer than the atmosphere, but that’s owed to the fact that the shirt’s materials also have properties that absorb heat; that has nothing to do with its color.

When we factor in research that demonstrates how the average runner’s per-mile pace slows by at least four seconds for every two degrees the temperature climbs above 59 degrees Fahrenheit, we have to consider that a poor choice of running attire dictated solely by color selection might have slowed a runner down by up to 46 seconds per mile in the circumstances we just described.

This can’t be right. If dark colors get so warm, why do some desert dwellers intentionally wear thick, dark clothing?

That’s a fair question, but it’s also one that can be easily dismissed once you know the science behind it. Apparently, the thickness of clothing ultimately prevents the light it absorbs from being transferred directly to your skin. The exterior of a thick, black robe will undoubtedly become extremely hot, but if the heat remains on the surface of the garment and never reaches the skin, it won’t significantly alter the wearer’s comfort level.

Does the color I’m wearing make things warmer if there’s no light?

No, because in the absence of light, there is no energy source to either reflect, or to absorb and convert into heat. Still, it’s essentially a moot point. If you’re running at night time when the light is low and the temperature is at its nadir, it theoretically doesn’t matter what color you wear, although it might be best to wear bright colors so that drivers can spot you while you’re jogging along the street

All in all, I can’t say that I can see someone being too concerned about how much light their clothes are absorbing from the stars.

But what if I’m indoors and away from the sun?

That’s an excellent question with a simple answer: The laws of thermodynamics still apply when the light being generated is from an artificial source, because it’s still energy. Energy isn’t created or destroyed; it just changes its form. Ask anyone who has ever been under the bright lights of a theater stage whether or not those lights are hot. Better yet, ask Billy Joel, who famously referred to it as “the white-hot spotlight” in the song “Big Shot.” 

The temperature of your LA Fitness is completely regulated, and the bulbs from the high ceilings are shining down on you at such a great distance that their effects are subdued. So, yes, your dark clothes will still get warmer than your white clothes in that controlled environment, but probably not to any noticeable degree.

So what should I wear?

Use your knowledge of lucency to your advantage. If you’re going for a run outside and you want to maximize your warmth, wear dark clothing. If you want to mitigate the extent to which light raises your body’s heat level, wear white. If you have some serious performance goals in mind, and you plan to be double-checking the pace calculator of your watch as you await the start of the Boston Marathon next October, then it’s probably worth accounting for the toll that light absorption by your clothing will extract from your workout. 

Otherwise, the only thermodynamic law you should be focusing on is how to transform the potential energy contained in your Bojangles cheat meal into calories that are burned through the pedaling of an elliptical.